Offshore wind turbines are being developed that instead of resting on fixed-bottom support structures have a floating support structure.
Several configurations have been proposed for the floating or buoyancy structures: many of these employ floater elements in the form of hollow floater tanks that in use are arranged substantially below the mean sea level and provide a buoyancy force to support the wind turbine. Ballasts in the floater and/or mooring lines anchored to the seabed are provided for achieving stability.
In some of these floating wind turbines, the buoyancy structure is designed to provide an excess buoyancy force and is maintained floating under the mean sea level by taut mooring lines tensioned by the excess buoyancy force.
For example, concepts have been developed such as the “Taught Leg Buoy” (TLB) floating wind turbine, with a slender cylindrical buoy and two sets of tensioned mooring lines, inclined relative to the seabed and connected to gravity anchors and to the buoy; or such as the “Tension Leg Platform” (TLP) floating wind turbine, in which the tensioned mooring lines are substantially vertical and are connected between gravity anchors on the seabed and arms or braces extending radially outwards with respect to the vertical axis of the wind turbine. The TLP arms may be part of the buoyancy structure, for example in the form of hollow spokes that extend radially outward from a hollow central hub, or may be arranged above the sea level, in which case the buoy may be a slender cylindrical tank like in the TLB concept.
The buoyancy structures of floating wind turbines are subject to several loads, such as for example the weight of the wind turbine itself, impacts, forces exerted by waves, currents and tides, and also aerodynamic forces associated to the wind, rotor rotation, etc. These loads, if not sufficiently counteracted, can cause destabilization of the wind turbine and can consequently negatively affect to its operation.
In general, the previously commented structures for offshore wind turbines seem to achieve a certain stabilization of the wind turbine. However, said stabilization may not be ensured enough in some particular situations. For example, wave loading and related low frequency effects may not be significantly counteracted according to the principles attributed to these buoyancy structures. Moreover, dampening of undesired rotations of the wind turbine, especially about a longitudinal axis of the tower (which can be called yawing movements), is not particularly considered by the prior art systems. These yawing movements may penalize the performance of the wind turbine and cause fatigue loads that can finally damage some components of the wind turbine.
Furthermore, any wind turbine normally comprises a pitch system for suitably pitching the blades. It is known that pitch systems can be used to reduce nacelle fore-aft oscillations induced by aerodynamic loads at the first tower mode by adding a collective pitch demand to the general collective pitch demand for rotor/generator speed control. This demand may be typically in phase with the tower fore-aft movement at this first oscillation mode. This way, the thrust force exerted on the nacelle by the wind is changed in such a way that the oscillation is damped. This can be done since the actuation for tower damping and speed control are pretty independent because they act in different frequencies ranges (typically 0-0.1 Hz for speed control and 0.3-0.4 Hz for tower damping).